Modulation system



July 20, 1948. u c soN 2,445,6l8

MODULATICVJN SYSTEM Filed April 1, 1943 MAMMA MAMMA .VVVVVVVK V VVVVVVVK VVVVVVVX H J JL MMMM MMMAMM MMM. MMMM MMMMM x MMM John f). Hu ?these/z.

ATTORNE Patented July 20, 1948 2,445,61s MODULATION SYSTEM John A. Hutcheson, Baltimore, Md., assignor to *Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of Pennsylvania' Application April 1, 1943, Serial No.- 481,383

This invention relates to 'communication systems and more particularly to systems emploing ultra-high-frequency carrier Waves.

With the event ofnew techniques' in producing ultra-high-frequency energy of relatively high power, various problems arose' in utiliz'ing this energy for communication purposes and'particulary forrvoice frequency transmission. The major problem concerns the modulation 'of ultra-highfrequency energy in such manner that the modulation signal may be transmitted without appreciabl distortion. In speaking of ultra-high-frequency currents, it is understood that the energy transmitted has a frequency inthe neighborhood of 500 megacycles up to'several thousand megacycles. At these frequencies, various operational difficulties have been encountered in view of the critical conditions existing in th'e apparatus'produeing such high -frequ'enc currents 'as'well as'in the transmission thereof at a distance with satisfactory efiiciency. i 7 Z As'a source for ultra-high fre'quency'currents, vacuum tubes have been developed which form a complete toscillating system* and operate on' the electron velocity principal; "As an example,

tubes of this type are described "in United States Patent 2,242,275-to R. H. Varian, `issued May 20, 1941. They are also referred toas electrical' converters which produce osillationsbyhsing an electron beam by 'means of space-resonant devices which form' highly fiicientitank circuits for the oscillator. The electrons are alternately accelerated and decelerated at the frequency of oscillation, and ultimately the energy of the electron beam is converted into electromagnetic field energy. n

The present invention discloses a method and means for modulating the ultra-high-frequency energy produced by tubesof'the characterabove described. While it is' particularly advantageous for modulation of the 'output energy of electron velocity actuated tubes, it may also be applied to oscillators of' .various kinds operating' on the magnetron principleor of the vacuum tube feedback type.

The primary object of the invention is to pro-` vide a modulation system 'which is adaptable to control the source of ultra-high-frequency energy in such manner that the transmitted wave shall be modified in accordance With the intelligence 4 claims. (Ci. 179-4715) er for efficient modulation of ultra-high-frequency nergy of relatively high power without introducing distortion of the carrier frequency.

Other features and advantages will be apparent from the following description of the invention pointed out in particularity by the appended claims and taken in connection with the accompanying drawing in which:

Figure 1 shows a schematic circuit arrangement of the modulation system applied to an ultrahigh-frequency transmitter in the form of an electron velocity actuated type tube.

Figs. 2, 3, 4 and 5 illustrate 'by means of curves the frequency relations of the carrier andmodulation frequencies, and the characteristic of the resultanthigh-frequency energy transmitted.

Fig. 6 shows a simple schematic circuit of a suitable receiver for the communication system herein described.

In any modulation system in connection with high frequency energy, the difficulties for undistorted transmission of the carrier wave reside in the particular characteristics of the vacuum tubes which necessarily play an important part in all high-frequency apparatus. The modulation characteristic of vacuum tubes is seldom a straight line, and when the operating voltage applied to certain electrodes of the tube is varied in accordance with a modulating signal, the radio frequency output does not follow the magnitude changes in exact proportion with the variations du to modulation. Particularly is this true in connection with the electron velocity actuated type tubes above referred to, since their characteristics do not allow direct application of a modulation signal to any of the vacuum tube elements. When such a tube is modulated by impressing a sinusoidal potential between itsanode and cathode, the output of the tube will have an envelope of squared or stepped wave form. While the distortion encountered in modulating such tubes is of no consequence when the tube is used for telegraphic communication, it prevents the use of the tube in modulation systems for telephonic* communication. The present system about to be described provides a method of modulating tubes which are characterized by a nonlinear relation between radio frequency output and modulation signal input.

Referring to Fig. 1, the source of ultra-highfrequency energy is schematically indicated by the electron velocity actuated tube l. The invention does not concern the particular form of the source of ultra-high-frequency energy, and the tube l is chosen here merely by' way of example. Those skilled in the art are familiar with the various types of tubes now available for generating ultra-high frequencies, and no comprehensive discussion of the principles of operation of such tubes is believed to be necessary here. The principal elements of the tube l include the cathode 2, the control grid 3 and the anode 4. In order to simplify the illustration, the operating potentials for energization of the various components of the system are shown here by bat teries. It is, of course, understood that any suitable power supply may replace these batteries which are merely symbols of operating voltage sources. The battery 5 provides the high voltage required .between cathode and anode of the tube l. It is assumed that the cathode is suitably energized to have the required temperature for electron emission. The oscillating circuit within the tube includes the space-resonant devices 6 and l which are excited by the beam of electrons projected from the cathode 2 to the anode 4. The ultra-high-frequency energy is conducted from a coupling loop 8 terminating in the space resonator 'I, and is conducted by means of the wave guide 'a to a suitable radiator Il located in a reflector. '4. The control electrode 3 is provided with the required operating potential from the voltage source shown here 'by the battery '2 through a grid load resistor '3. The description so far given was directed solely to the source of ultra-high-frequency energy which is part of the communication system. This source is shown to be a particular type of vacuum tube, and it is assumed that the operating elements of this tube are suitably proportioned and the operating voltages are so distributed that the tube l will operate to generate an ultra-high-frequen-cy Wave which can be effectively radiated through space. It is to be noted for a thorough understanding of the modulation system herein disclosed that the operating characteristic of the tube I is such that the grid electrode 3 is effective for controlling the operation of the tube in such manner that within certain values of grid potential the tube will function as an oscillator and at a discrete value it will cease to function as such. In other words, the operation of the tube may be intermittent depending upon the particular value of operating potential applied to its control grid 3.

The modulation circuit comprises a vacuum tube is in a conventional oscillator circuit for producing high-frequency oscillations. The anode l" thereof connects through a portion of the inductance '8 to the cathode '9 including the operating voltage source shown here by the battery 20. The grid electrode 2' is coupled through a portion of the inductance IS by means of the condenser 22 and returns through a suitable leak resistor 23 to the .cathode 19. Condensers 25 and 26 in series are effectively connected across the terminals of the inductance '3, the junction point of the condensers 25 and 26 being returned to the cathode '9. The inductance '8 together with the condensers 25 and 26 form a frequency determining circuit for the oscillator tube 16. The frequency of its operation is determined by the particular value assigned to the inductance '8: and the condensers 25 and 26. connected to the oscillator circuit '6 is the modulator tube 28 having an anode 29, a control electrode 30 and a cathode 3 l. The anode 29 connects directly to the anode l'! of the oscillator tube IG and thereby receives operating potential for its anode circuit from the same voltage source, namely, the bat- 4 tery 20. The cathode 3' is Similarly connected to ground as is the cathode l9 of the tube IG. The grid 30 returns to the cathode through the resistor 33 and the secondary winding 34 of the modulation input transformer 35. A suitable source of grid bias potential is included in the circuit and is represented here by the battery 36. The grid circuit is coupled to the anode circuit by means of the condenser 3". By virtue of this connection, there is formed a quadrature circuit whereby the grid electrode 30 is supplied with energy from the .plate circuit through a resistance capacity network. The grid voltage appearing will be 'effectively at a phase shift with respect to the .plate voltage. The anode resistance of the tube 28 represents also a parallel load for the tank circuit of the oscillator. A circuit of this type is also known as an electronic reactance circuit, the effective reactance value of which may be controlled by varying the bias potential ofthe grid 30, which, in turn, will vary the effective reactance of the vacuum tube 28. This will react on the oscillator circuit in effect acting as a Variable inductance or capacity in shunt with the tank circuit. The Variation in the grid bias for control purposes is obtained by the modulation signal which is applied to the primary winding 38 of the transformer 35 from the microphone 40 which alters the current flow through this winding from 'the battery 41.

The operation of reactance .circuits are well known in the art and have been described in great detail in the Proceedings of I. R. E., Volume 23, by Travis. For the purpose of explaining the operation of the modulation system, it should be understood that the frequency of the oscillator IS may be varied within a certain band of frequencies by variations of grid potential of the control tube 28. Variation is eifected by the signalling current produced by the microphone 40 in the secondary winding 34 of the transformer 35. The frequency of the oscillator will undergo a deviation in proportion to the intensity of the signalling currents and this deviation will occur at a rate of the signal frequency. In order that the oscillator frequency shall control the generation or transmission of ultra-high frequency, a coupling is provided between the oscillator output and the control grid 3 of the tube l by means of the 'condenser 42. In this manner the Output voltage of the oscillator is applied, and utilized to vary the potential of the grid 3 and trigger the action of the ultra-high-frequency generator tube l as will be described in connection with the operation of the modulation system. i

The communication system so far described included the transmitting portion which, has the following basic components: (l) a generator of ultra-high-frequency radiation; (2) a source of high-frequency energy adaptedto control the operation-of the generator; and (3) a control circuit for the high-frequency oscillator to vary the oscillation frequency thereof in accordance, [with variations of a modulating signal which is impressed on the modulation circuit. The opera,- tion of the transmitting circuit will be better understood by reference tothe curves ,of :'Figs. 2, 3, 4 and 5. The sine wave of Fig. 2 shows. the Output of the oscillator tube .IG and the curves of Fig. 3 illustrate asine Wave of much higher frequency, namely the output 'of the ultra-high-frequency transmitter emitted from the radiator H. The two curves just mentioned show the condition of signal transmission when there is no modulation of thetransmitter, that is; when the tube I.

microphone 40 is 'not energized Figsf; 4 ar'd show curves similar to those of 2 and 3, respectively, illustrating the oscillator output'with frequencyvariations of the oscillator within a predetermined band of frequencies, and the corresponding wave trains emitted by the radiator Il.

In connectionwith the operation of the transmitting system, it should be borne in mind that the ultra-high-frequency generator tube l can be controlled as to its operation in -suchmanner that at and below a critical positive potential with respect to itsoathode the gridelectrode '3 will function to cease operation entirely of the Under such conditions no oscillations are generated consequently no transmission takes place, whereas, above this critical potential in the opposite sense and therebeyond, the grid 3 will not impede the electron beam and the tube l will generate ultra-l'igh-frequency 'energy which will be radiated in the form of a carrier 'wave by the radiator H. The output voltage of the oscillator tube '6 is calculated to be of such value that at one half-cycle of its operation the voltage impressed on the grid 3 will be subtractive from thebias voltage of the battery '2 to such an extent that cut-off point is reached and the tube will cease to function. On the positive half-cycle of the output wave bf the tube '6,5 on

- the other hand, normal operation of the tube l willbe obtained The frequency of the os'cillator tube '6 is chosen to be a suitable value lower than the ultra-high-frequency transmitted, The particular value of the frequency chosen will'depend largely on practical consideration as to receiver designsinceit is one of the salientfeatures of the system that conventional receiver circuits may be used.

For practical purposes the following relation may be chosen as an example. Let us assume that the tube l will generate a 3000-megacycle carrier frequency. The frequency of the oscillater tube '6 may be two megacycles per second in the absence of a modulation signal. If the oscillator '6 is operating, a voltage of two megacycles frequency will appear on the grid 3 of the ultra-high generator tube l. This will cause the latter to start and stop its oscillations at the rate of two megacycles per second. Therefore, the output of the ultra-high-frequency generator I will consist of a groups of 3000 megacycles radio frequency oscillations each group being approximately of a /4 microsecond duration followed by a period of equal duration when the generator tube I will not operate. In other words, there will be two million such groups of oscillations each second. Referring to Fig. 2, it is seen that at the positive half-cycle of the output voltage of the oscillator IE there is a corresponding group I of ultra-hig-h-frequency Waves shown in Fig. 3. At the negative half-cycle of the output wave of Fig. 2, the oscillations of the generator tube l will cease and no energy is radiated at all. This is repeated at each succeeding half-cycle of the oscillator frequency, resulting in groups of wave trains radiated. The succeeding groups I, II, and III of ultra-highfrequency radiation shown in Fig. 3 intend to convey the idea of successive wave trains. The exact numerlcal ratio of oscillations for one halfcycle with respect to the control frequency of the oscillator tube '6 is not intended to be shown here in order to make the illustration practical and avoid over crowding of the lines representing each wave train. Knowing the duration of each operating half-cycle, it can easily be w in turn vary the duration per unit time ofthe output wavetrains of the generator' l. In Fig. 4; the-output frequency of the oscillator '6 is shown -to' vary 'from 'the `two-megacycle mid frequency to therlower limit of 1.9 megacycles shown by the second output w'ave to 2.1 megacycles shown by the third output wave. The resultant ultra-high-frequency energy in the first group h'as` a" time duration of the positive half-cycle' of the two-megacyclecontrol frequency, whereas in the secondgroup the time duration is extended in view of the` fact that the positive half cycle of the control frequency is of a frequency lower 'than before. The 'third group illustrates -the other'limit of the frequency shift showing a group of the shortest duration per unit time corresp'onding to the positive half-cycle of a 2.1 megacyclecontrol^`frequency; I r

The modulation system' as desorlbed permits operation at ultra-high frequencies by allowing transmission of ultra-high-frequen'cy energy-of substantially constant amplitude at a rate which varies in accordance with a' predetermine d frequency.- The latter controlled as to frequency deviation impresses the modulation component on the carrier in the form of varying groups of ultra-high-frequency energy having a basic rate determined by the generator tube l. Analyzing the output wave, it is found that it contains a :component of the fundamental frequency of the control oscillator '6 and that this component is frequency-modulated in accordance with the modulation signal. In order to receive this signal, a simple rectifier may be used the output of which can be amplified by an intermediate frequency amplifier which has a band-width correresponding to the range of frequencies of the modulating signal. The range in accordance with the example herein given would have to be between 1.9 and 2.1 megacycles.

A simple receiving circuit is shown in Fig. 6 comprising the receiving antenna 50, from which the signal is transmitted through a wave guide 5' to a crystal detector 52, shown here in a block diagram form representing any conventionl ul tra-high-frequency crystal receiver. The output of the crystal 52 is connected through suitable coupling means to a conventional intermediate frequency amplifier 54 shown also by block diagram. The output of the amplifier is impressed on a suitable frequency discriminator circuit illustratedhere by the diode tubes 55 and 56 fed by the coupling transformer 5". The output of the discriminator will be a reproduction of the modulation signal impressed on the transformer 35 from the source 40. The signal may be applied to suitable audio frequency amplifiers of which one stage is shown here by the vacuum tube 59 having in its output circuit a telephone receiver 50.

I claim as my invention:

1. In a communication system operating at ultia-high frequencies, a source of ultra-high carrier frequency energy comprising an oscillator of the electron velocity actuated space-resonant type, a source of high. frequency energy of a frequency lower than said carrier frequency, means for triggering the action of said oscillator at a rate determined by said source of high frequency energy, means for im-pressing modulations comprising means for shifting the frequency of said last-mentioned source in. accordance with the amplitude of the signal to be transmitted. and at a rate of said signal frequency,

2. In a communication system operating at ultra-high frequencies, a source of Ultra-high carrier frequency energy compris ing aan oscillator of the electron velocity actuated space-resonant type, a source of high frequency energy of a frequency lower than said carrier frequency, means for controlling the operation of said oscillator at intervals of unit time determined by the instam taneou-s frequency of said high frequency source, means for impressing modulations comprising means for shif ting the frequency of said lastmentioned source in accondancewith the amplitude of the signal to be transmitted and at a rate of said signal frequency.

3. In a communication system operating at ultra-high frequencies, a source of ultra-high carrier frequency energy comprising an oscillator of the electron velocity actuated space-resonant type, a, source of high frequency energy of a frequency lower than said carrier frequency, means for controlling the operation of said oscillator at intervals of unit time determined by the semicyclic time base of the instantaneous frequency of said high 'frequency source, means for impressing modulations comprising means for shift- 'ing the frequency of said last-mentioned source in accordance with the amplitude of the signal to be transmitted and at a rate of said signal frequency.

4. In a modulation system for ultra-high frequency carrier transmission, a source of uitrahigh frequency energy in the for-m of an electron velocity actuated space-resonant type tube having principal operating electrodes and a control electrode effective in controlling the operation of said tube, a sec-0nd source of high-frequency energy comprising a 'vacuum tube oscillator a frequency determining circuit for said oscillator, circuit means for coupling the Output of said oscillator to said control electrode, a source of modulation frequency currents, and means for impressing the modulation frequencies of said source upon said frequency determining circuit for effecting variation of frequency of said oscillator.

JOHN A. HUTCHE SON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,069,538 Philpott Feb. 2, 1937 .2,113,214 Luck Apr. 5, 1938 2,203,75'0 Sherman June 11, 1940 2,252,293 Ohl Aug. 12, 1941 2,266,40-1 Reeves Dec. 16, 1941 2,280,026 Brown Apr. 14, 1942 2,293,387 Haeff Aug. 18, 1942 2,316,123 Scheaffer Apr. 6, 1943 

